The violence that turns a small-town protest into a fiery national spectacle like the one that has played out this month in Missouri is often unwittingly provoked by police, according to researchers at UC Berkeley.

The research team, which studied clashes between police and activists during the Occupy movement three years ago, found that protests tend to turn violent when officers use aggressive tactics, such as approaching demonstrators in riot gear or lining up in military-like formations.

Recent events in Ferguson, Mo., are a good example, the study’s lead researcher said. For nearly two weeks, activists angered by a white police officer’s fatal shooting of an unarmed black teenager have ratcheted up their protests when confronted by heavily armed police forces.

“Everything starts to turn bad when you see a police officer come out of an SUV and he’s carrying an AR-15,” said Nick Adams, a sociologist and fellow at UC Berkeley’s Institute for Data Science who leads the Deciding Force Project. “It just upsets the crowd.”